Method for post-chemical mechanical polishing cleaning

ABSTRACT

A method for cleaning residue of alignment marks on a CMP polished wafer. The method includes application of a strong acid or oxygen plasma to the wafer to remove the residue.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method for cleaning residue of alignment marks, and in particular to a method that is used after chemical mechanical polishing to clean alignment marks.

[0003] 2. Description of the Related Art

[0004] In a semiconductor process, every single step must be carried out precisely at predetermined positions on wafers to maintain product quality. In order to do so, optical alignment marks are used for correct positioning on semiconductor substrates. Generally, optical alignment marks are specific patterns formed on semiconductor substrates. During illumination of the alignment marks, refraction of light is caused by the height differences of these alignment marks. Wafers are then adjusted to the correct position according to the refraction. However, chemical mechanical polishing (CMP), frequently used during semiconductor process, causes residue on the alignment marks. In order to clearly illustrate the residue problem, FIGS. 2, and 3A-3C are referred to in the following explanation. FIG. 2 illustrates SEM of the residue on the alignment marks. FIG. 3A illustrates an enlarged area of the residue on the alignment marks of FIG. 2. FIG. 3B illustrates an enlarged area of the residue on the alignment marks of FIG. 3A. FIG. 3C illustrates an enlarged area of the residue on the alignment marks of FIG. 3B. From the figures, it is observed that residue, i.e. black spots in the figures, are organic substances deposited in the trenches of the alignment marks. This causes incorrect positioning at later stages of lithography. As a result, the residue must be removed to obtain correct positioning later.

[0005] Conventionally, either hydrogen fluoride (HF) or ammonium hydrogen peroxide mixture (AMP) is used for cleaning after CMP. However, it cannot effectively remove organic residue. Furthermore, it produces adverse effects, such as reaction with metal, thereby damaging wafers. Overall production yields are thus seriously affected.

SUMMARY OF THE INVENTION

[0006] In order to solve the above problems, an object of the invention is to provide a method for cleaning alignment marks that removes organic residue from the alignment marks by strong acid or oxygen plasma.

[0007] The method removes residue from alignment marks on a wafer. The method includes application of strong acid or oxygen plasma, thereby removing the residue. Since the residue contains hydrocarbons, CO₂ is produced when reacted with oxygen plasma, thus removal of residue is achieved.

[0008] Examples of strong acids preferably used in this method include sulfuric acid, nitric acid, or hydrogen chloride. Wafers in the invention are specific wafers that have been polished by chemical mechanical polishing (CMP). The oxygen plasma applied to residue on the alignment marks is preferably generated in dry etching apparatus.

[0009] The method for cleaning alignment marks is also applicable as a post-chemical mechanical polishing (CMP) cleaning method. The method includes application of strong acid or oxygen plasma to CMP polished wafers to cause chemical reactions of the residue with the strong acid or oxygen plasma, thereby removing the residue.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:

[0011]FIG. 1 illustrates a chemical mechanical polishing apparatus.

[0012]FIG. 2 is a SEM of the residue on the alignment marks.

[0013]FIG. 3A is a SEM showing an enlarged area of the residue on the alignment marks of FIG. 2.

[0014]FIG. 3B is a SEM showing an enlarged area of the residue on the alignment marks of FIG. 3A.

[0015]FIG. 3C is a SEM showing an enlarged area of the residue on the alignment marks of FIG. 3B.

[0016]FIG. 4 is a flowchart of the method for cleaning alignment marks according to the invention.

[0017]FIG. 5 is a SEM illustrating the wafer after cleaning by the method according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0018] The method for cleaning alignment marks provided in the present invention is specifically for wafers that have been polished by CMP. It is also a post-CMP cleaning method for removing residue from alignment marks. Conventional CMP is carried out by cooperation of polishing pad and suitable slurry to planarize the surface of wafers. Polishing modes include rotary and linear, both of which are applicable for the present invention.

[0019]FIG. 1 illustrates a chemical mechanical polishing apparatus, wherein 1 represents CMP apparatus, 11 represents a spinning table, 13 represents the polishing pad, 15 represents input of slurry, 16 represents slurry, 18 represents wafer bearer, and 20 represents wafers. In CMP, a wafer 20 is attached to a vacuum disk, and the spinning table 10 rotates in a direction shown by the number 22. Polishing between wafer 20, slurry 16 and the polishing pad 12 is therefore induced by the rotation of the spinning table 10. Micro-sized polishing powder is often used in CMP, such as SiO₂, Al₂O₃, CeO₂, or ZrO₂. Examples of chemical assistants are pH buffer, such as KOH, NH₄OH, or organic acids, oxidizing agents, such as peroxide, iron nitrate, or potassium iodate, and surfactants.

[0020] In FIG. 4, a CMP polished wafer having residue on its alignment marks is provided. A strong acid is then applied to the wafer as step S10. In this example, solution containing sulfuric acid (concentration of 18M) is used. After the sulfuric acid solution reacts with the residue, the residue is removed. Apart from sulfuric acid, other acids, such as nitric acid and hydrogen chloride are applicable as well.

[0021] A similar procedure in Example 1 can also be performed, with the difference that the wafer having residue on its alignment marks is placed in a dry etching apparatus for the application of oxygen plasma, as step S10 in FIG. 4. Parameters for application of oxygen plasma include pressure of 1˜5 torr and flowrate of 2500˜3500 sccm. Residue (organic substances containing hydrocarbon) then reacts with the oxygen plasma to form CO₂, thereby removing the residue from the alignment marks.

[0022] According to the method for cleaning alignment marks provided in the present invention, the result is shown in FIG. 5, a SEM showing the cleaned alignment mark. Clearly, residue is successfully removed. Consequently, the object of the invention is achieved.

[0023] According to the method for cleaning alignment marks provided in the present invention, removal of residue from the alignment marks is accomplished using strong acid or oxygen plasma. Problems associated with organic substances covering alignment marks and their effects on semiconductor quality are solved accordingly. Conventional damage occurring to wafers when using HF or AMP to clean wafers is avoided as well.

[0024] While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements. 

What is claimed is:
 1. A method for cleaning residue of alignment marks on a wafer, comprising applying a strong acid to the wafer to remove the residue.
 2. A method for cleaning residue of alignment marks on a wafer, comprising applying oxygen plasma to the wafer to remove the residue.
 3. The method as claimed in claim 1, wherein the wafer has been polished by chemical mechanical polishing.
 4. The method as claimed in claim 2, wherein the wafer has been polished by chemical mechanical polishing.
 5. The method as claimed in claim 1, wherein the strong acid is sulfuric acid, nitric acid or hydrogen chloride.
 6. The method as claimed in claim 2, wherein the oxygen plasma is generated in a dry etcher.
 7. A post-chemical mechanical polishing cleaning method, used to remove residue from alignment marks on a CMP polished wafer, the method comprising applying a strong acid to the wafer to remove the residue.
 8. A post-chemical mechanical polishing cleaning method, used to remove residue from alignment marks on a CMP polished wafer, the method comprising applying oxygen plasma to the wafer to remove the residue.
 9. The method as claimed in claim 7, wherein the strong acid is sulfuric acid, nitric acid or hydrogen chloride.
 10. The method as claimed in claim 8, wherein the oxygen plasma is generated in a dry etcher. 